1. Field of the Invention
This invention relates to the focusing of zoom lenses, and more particularly to a lens design for improving the stabilization of aberration correction when the zoom lens, beginning with a lens group of negative refractive power is being focused.
2. Description of the Prior Art
In the past, a wide variety of zoom lenses, beginning with a component of negative refractive power and, in particular, with the inclusion of another component of positive refractive power on the image side of the former, so that these two components are made axially move in differential relation to effect zooming, have been designed, but most of these zoom lenses have the focusing provision at all the elements of the negative component.
By the recent trend of minimizing the bulk and size of this type lens system, the refractive powers of such two, or negative and positive, components are both caused to increase. For a further advance in compactness, an additional component of negative power is apt to be used and is arranged in rear of the positive component and remains stationary during zooming.
And, in this case also, the negative and positive components movable for zooming are given further stronger refractive powers.
With respect to focusing, such an increase in the refractive power of the front negative component has great advantages in that the total focusing movement, whose terminal ends correspond to infinite and minimum object distances, is reduced and that it can contribute to an improvement in the compactness of the lens system as a whole, but leads to a prominent variation with focusing of aberrations, particularly when in telephoto positions and thereabout. This is intensified as the relative aperture increases, the focal length increases and the minimum object distance shortens, resulting in deterioration of aberrations, particularly spherical aberration (over-corrected near or at the minimum object distance), which have been very difficult to correct.
Meanwhile, in the art of fixed focal length lenses, to maintain some of the aberrations stable throughout the focusing range, a focusing method of bodily moving the lens while simultaneously varying one or two lens separation or separations has been put into practice as is known in U.S. Pat. No. 3,748,021, for example. If this method is applied to zoom lenses not in an appropriate way, however, a large increase in the necessary number of lens elements is called for, and the operating mechanism is made complicated in structure.